Energy efficiency is an increasingly important requirement for devices that communicate in a radio network, and in particular for battery-powered mobile communication devices. Mobile communication devices are also referred to as User Equipment (UE) in the context of a cellular telecommunication network according to Universal Mobile Telecommunications System (UMTS), Long Term Evolution (LTE) or LTE-Advanced (each of which standardized by the 3rd Generating Partnership Project, or 3GPP). Regarding mobile communication devices, long stand-by times are a criterion of user satisfaction. From the perspective of stationary communication devices, network providers also have gained interest in energy efficient “green” base stations as usage and density of radio networks increase.
In order to achieve energy efficiency and long stand-by times, communication protocols have been developed that schedule idle periods, in which the communication device enters a period of reduced power (also referred to as power-save mode). As a mobile communication example, the mobile communication device and the base station negotiate phases, in which data transfer occurs. During other times, the UE turns its receiver and transmitter off and enters a period of reduced power. As another example of mobile communication, the UE in a disconnected state may enter a period of reduced power, which is periodically interrupted for receiving a paging channel indicating an incoming call or a Short Message Service (SMS). Also in a connected state having a transmission structure with time slots including headers that define an address of the transmission, the UE may limit its reception period to the headers in each of the time slots in order to decide whether or not the transmission is relevant. In addition, a base station comprising a pool of Digital Signal Processors (DSPs) may switch off some or all of the DSPs depending on a current workload.
US 2007/0135081 A1 discloses a mobile communication device that comprises an oscillator providing accuracy for synchronized communication parameters in an active mode and a Real Time Clock (RTC) used as a frequency standard during reduced power operation. While power consumption of the RTC is reduced compared to the oscillator, the precision of the RTC is low.
U.S. Pat. No. 6,453,181 B1 suggests using a dedicated sleep clock for a sleep period. The mobile communication device receives signals from a base station having high timing accuracy. In order to compensate for a drift of the sleep clock, an initial frequency of the sleep clock is determined following power-up of the mobile communication device. Furthermore, during a slotted mode of operation, a dynamic frequency error compensation factor is updated by determining an amount of timing slew between the mobile communication device and the base state.
In prior art devices, reasonable accuracy of the radio time base is thus achieved after returning from a power-save period by comparing an RTC with an oscillator signal or with a base station signal, each of which being time consuming to achieve sufficient accuracy and lasting up to one second. The most recent comparison is used to restore the radio time base. Such a conventional comparison thus takes a relatively long time (e.g., compared to the power-save period) or may prevent a further increase in accuracy that would take even longer. Furthermore, in order to account for, e.g., temperature variations, the comparison has to be updated, which still further increases time and power consumption, and may eventually lead to the need of keeping the oscillator permanently active.
If the accuracy was reduced, errors in the time base after the power-save period could lead to a loss of the paging signal as the correct time window for radio reception is missed. One might consider compensating for the reduced accuracy by waking-up the UE an additional time before the paging window. The additional time may correspond to an uncertainty in the time base or may allow for additional processing to achieve time synchronization based on the received radio signal. Such compensation, however, entails an increase in power consumption due to prolonged radio reception time.